The structure of a front end of a system required to commonly use a single antenna in transmission/reception is diversified with the development of wireless communication systems. For example, a structure capable of supporting both transmission/reception combination schemes through a frequency division demultiplexing (FDD) and a time division demultiplexing (TDD) is required and a structure in which a communication (for example, an inter-matter communication, M2M) between FDD systems having the same transmission/reception frequencies is supported is also required. In this case, the isolation between transmission/reception also needs to be significantly ensured.
Meanwhile, a front end of the wireless communication system, which is connected with an antenna transfers a high-power transmission signal generated in the system to the antenna and transfers a reception signal received from the antenna through a reception route, and in this case, it is important to reduce loss as possible. The front end is basically required to be configured by an element suitable for high power in order to process a high-power transmission signal.
In order to commonly use the single antenna in transmission/reception, the front end of the wireless communication system uses the time division demultiplexing (TDD) scheme and the frequency division demultiplexing (FDD) scheme. In this case, the former, which is a scheme using the same frequency in transmission and reception, and alternately operating by temporally dividing transmission/reception to use the single antenna, switches a transmission mode and a reception mode using a switch. In the latter, as transmission and reception are temporally simultaneously performed by differentiating transmission and reception frequencies, the transmission signal and the reception signal are combined by a single port while the transmission signal and the reception signal are divided and isolated by different filters.
In FIG. 1A, transmission/reception signals having the same carrier frequency are temporally divided to be connected with the antenna by using a switch in order to combine transmission and reception in the TDD scheme. That is, a transmission signal Tx is input into a switch 103 through a first band pass filter (hereinafter, a ‘BPF’) 101 and transmitted to a wireless space through an antenna 104 as the switch is switched to the transmission mode.
On the contrary, a reception signal Rx received through the antenna 104 is input into a second BPF 102 as the switch 103 is switched to the reception mode, and received while being band-pass filtered in a corresponding band by the second BPF 102.
In FIG. 1B, a duplexer is used to combine transmission and reception in the FDD scheme. The duplexer divides and combines the transmission and reception signals having different carrier frequencies by using a filter 105 and is connected to the antenna 106 temporally simultaneously.
In the TDD scheme, as a structure in which the transmission mode and the reception mode are divided by the switch, various transmission/reception frequencies can be configured according to a configuration of the filter on transmission/reception route. This is the structure in which the communication (for example, the inter-matter communication, M2M) between the systems having the same transmission/reception frequencies is possible, but the communication is possible only in the TDD scheme and impossible in the FDD scheme in which transmission and reception operate simultaneously. A high-cost and high-power switch is required for high-power transmission.
FIG. 2 is a transmission/reception system using a single circulator 203. In FIG. 2, transmission is connected toward the antenna and reception is continuously connected to a reception route in the antenna by directionality of a signal regardless of the carrier frequencies of transmission and reception in FIG. 1 to support both the TDD and FDD schemes.
That is, the transmission/reception system of FIG. 2 uses a characteristic of a circulator of which an advance direction is determined according to the directionality of the signal. The transmission/reception system is configured such that the transmission signal is transferred to an antenna 204 through a first BPF 201 and the circulator 203 and the signal received by the antenna 204 is received through the circulator 203 and a second BPF 202. Therefore, the transmission and reception routes may be combined regardless of the frequency.
Various transmission/reception frequencies can be configured according to the configuration of the filter on the transmission and reception route, the communication (for example, the inter-matter communication, M2M) between the systems having the same transmission/reception frequencies can also be supported in both the TDD/FDD schemes, and the front-end is constituted by low-priced passive elements even in high-power transmission.
However, in the radio-frequency front end using the circulator, isolation between transmission and reception is determined by isolation of the circulator and the radio-frequency front end shows somewhat small isolation which is approximately 25 dB. Therefore, the high-power transmission influences the transmission route to influence the performance of the receiver.